Quantum entanglement occurs when physical properties of multiple quantum systems become related in a single quantum state that cannot be simply factored. Many quantum information systems and processes and particularly those that are measurement-based need the ability to entangle the quantum states of separated or remote quantum systems. One technique for entangling the states of remote quantum systems uses the interaction of photons with the quantum systems because photons can retain quantum coherence while traveling between remote quantum systems. However, these entanglement processes generally require the interacting photon from one quantum system to have a frequency that corresponds to the energy levels of the other quantum system and thus are intolerant of spectral diffusion of the optical transitions of quantum systems. For example, some entanglement processes do not work properly if the optical transitions of the quantum systems fluctuate in frequency by an amount about equal to or larger than the natural line width of the spontaneous photon emission spectrum from the quantum systems.
Intolerance for variation in the optical transition energies of quantum systems is a general problem for quantum-optical devices that are fabricated in a solid state structure, wafer, or chip. In particular, current wafer fabrication processes are subject to variations and defects that alter the performance of individual quantum devices, so that different quantum devices that are intended to have the same energy levels may actually have different energy levels and different transition energies. Also, in some solid-state quantum systems such as quantum dots and molecules, the frequencies of optical transitions can fluctuate, for example, due to fluctuating charge traps within a few tens of nanometers of the quantum systems. Spectral diffusion can be particularly severe when a quantum system is close to a surface or interface between different materials where charge may collect over time, and many solid-state quantum systems are in cavities with small mode volumes and must be close to a surface in order to efficiently interact with light.
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